Compounds of the type c-Fe2−xSnxO3 were prepared by adding aqueous ammonia to aqueous mixtures of iron(III) chloride hexahydrate, iron(II) chloride tetrahydrate, and tin(II) chloride until pH 7. The mixtures were boiled under reflux (3 h). The precipitates were removed by filtration, washed with 95% ethanol until no chloride ions could be detected in the washings by silver nitrate solution, and heated in air at 250 °C (12 h). Compounds of composition a- Fe2−xSnxO3 were prepared by precipitating aqueous mixtures of iron(III) chloride hexahydrate and tin(IV) chloride with aqueous ammonia and hydrothermally processing the precipitates  in a Teflon-lined autoclave at 200 °C and 15 atm pressure for 5 h. The products were removed by filtration and washed with 95% ethanol until no chloride ions were detected by silver nitrate solution. Details can be checked in J. Mater. Chem., 1999, 9, 223–226

nice and easy way is told by Sundeep Kumar Dhawan, if you do not find it proper for your condition, then let us know what you want.

Sir Kenneth M Towe, you pointed out very important point here. Atmospheric heating will not give well crystallized product, also not 100% yield, further heat treatment is needed, but drawback in 2nd stage heating is  synthered and agglomerated particles. however, high pressure heating at relatively low temperature yields well crystallized and well dispersed nanoparticles,  another question is the temperature - 200oC, it maybe a litle low, some adjustment may be needed, but order is the same. 

Dear Artavazd;

In hydrothermal process, heat treatment at 200°C is not a low temperature because of high pressure. Instead it is a relatively high temperature, may be equall to 1000°C in conventional process.